1. Field of the Invention
The invention relates to an ink jet recording head in which a part of a pressure chamber communicating with a nozzle opening is expanded and contracted by an actuator conducting flexural vibration, thereby ejecting ink drops through the nozzle opening.
2. Description of Related art
There are two types of ink jet recording heads, i.e., the piezoelectric vibration type in which ink is pressurized by mechanically deforming a pressure chamber, and the bubble jet type in which a heating element is disposed in a pressure chamber and ink is pressurized by an air bubble produced by heat of the heating element. Ink jet recording heads of the piezoelectric vibration type are classified into two categories, a first recording head using a piezoelectric vibrator which is axially deformed, and a second recording head using a piezoelectric vibrator which conducts flexural displacement. The first recording head can be driven at a high speed and performs recording at a high density, but requires a cutting operation for producing the piezoelectric vibrator, and a three-dimensional assembly operation for fixing the piezoelectric vibrator to the pressure chamber, thereby producing a problem in that an increased number of production steps are necessary. By contrast, in the second recording head, the piezoelectric vibrator has a membrane-like shape, and hence can be formed by baking the piezoelectric vibrator integrally with an elastic film constituting the pressure chamber. Consequently, the second recording head has a reduced number of production steps. However, the second recording head requires an area of a size sufficient to conduct flexural vibration so that the pressure chamber has a large width, thereby reducing the arrangement density.
In order to solve the problem of a recording head using flexural vibration, for example, Japanese Patent Publication (Kokai) No. HEI5-504740 discloses an ink jet recording head comprising: a substrate in which pressure chambers are formed in a single-crystal silicon substrate of a (110) lattice plane; and a nozzle plate in which a plurality of nozzle openings communicating with the pressure chambers are formed and which is fixed to one face of the substrate. The other face of the substrate is formed as a membrane which is elastically deformable. A driving portion is integrally disposed by forming a piezoelectric film on the surface of the membrane by a film formation method. The driving portion conducts flexural vibration so as to pressurize ink in the pressure chambers, thereby ejecting ink drops from the nozzle openings.
In the disclosed head, the pressure chambers, ink supply ports attached to the chambers, and, a reservoir are formed by conducting anisotropic etching on a single-crystal silicon wafer. Because of the characteristics of anisotropic etching, the pressure chambers are obliged to be arranged along a &lt;111&gt; lattice orientation of the single-crystal silicon wafer. This causes the wall face of the reservoir for supplying ink to the pressure chambers, to be formed on a (110) plane which is perpendicular to the &lt;111&gt; lattice orientation. However, it is very difficult to form the (110) plane by conducting anisotropic etching on a single-crystal silicon substrate. Therefore, a technique in which a wall face defining a reservoir is etched so as to be approximated by a continuum of minute (111) planes is employed.
In order to form minute (111) etched planes, patterns which are called compensating patterns and disclosed in, for example, Japanese Patent Publication (Kokai) No. HEI7-125198, must be formed so as to prevent the etching from being excessively conducted. The compensating patterns are gradually shortened as the etching of a single-crystal silicon wafer proceeds, and then formed into a sword-like shape which is necessary for minute (111) planes to remain at the completion of the etching. Consequently, the ink reservoir must have a width which is greater than at least the length of the compensating patterns, so that extra regions for the compensating patterns are required. This produces problems in that the size of the ink jet head is increased, and that an expensive wafer is wastefully consumed.
When an image such as a graphic image is to be printed, since dots must be formed at a high density, nozzle openings also are required to be arranged at a high density. As a result, very small ink drops, in the order of 10-30 ng per drop, are required to be ejected. In order to comply with these requirements, improvements such as reduced width pressure chambers, and partition walls of the pressure chambers having a reduced thickness must be produced in the substrate in which flow paths are to be formed. When the width of the fluid pressure chambers is reduced or when the partition walls are made thin, however, there arise further problems in that the ink flow in the pressure chambers is impeded, that air bubbles remain in the flow paths, and that the partition walls are easily deflected and crosstalk occurs, thereby impairing the printing quality.
Even when such requirements are fulfilled, a further requirement is produced as described below. A nozzle plate which closes one face of each pressure chamber is elastically contacted with and sealed by a capping member for preventing the flow paths from clogging, and rubbed with a cleaning member which is made of an elastic material such as rubber. Consequently, the nozzle plate must have a mechanical strength which can endure such operations. In order to ensure the strength, a metal plate member constituting the nozzle plate must have a thickness of 80 .mu.m or more. On the other hand, nozzle openings which can eject ink drops satisfying the above-mentioned requirements have a diameter of about 30 .mu.m on the ink ejection side. In the view point of problems which may be produced in processing, the diameter of nozzle openings on the pressure chamber side must be at least 70 .mu.m, preferably about 90 .mu.m. When pressure chambers are designed so as to have a reduced width so as to attain a higher arrangement density, therefore, nozzle openings are partly closed by partition walls of the pressure chambers, thereby producing a problem in that ink flow from the pressure chambers toward the nozzle openings is impeded.
Furthermore, a signal must be supplied to the driving portion without impeding the vibrating operation. Therefore, it is impossible to directly connect a cable to the driving portion. To comply with this, a structure must be employed in which a lead pattern elongating to the driving portion is formed on the surface of a vibrating plate and a cable is connected to the lead pattern at a position which is separated from the vibrating region. When the driving portion is formed by the above-mentioned film formation method, the level difference between the driving portion and the lead pattern must be made as small as possible so as to ensure the connection therebetween. Therefore, a countermeasure is taken in the following manner. The piezoelectric film constituting the driving portion is extended to the region where the lead pattern is to be formed, so as to serve as an insulating film for insulating a lower electrode. Thereafter, a lead pattern is formed on the surface of the piezoelectric film by vapor deposition or the like. However, this countermeasure has the following disadvantage. An electrostatic capacity of a value which is negligible in the view point of transmission of a signal is produced between upper and lower electrodes in the wiring region. This occurs because the piezoelectric film has originally a high specific dielectric constant and is very thin. The extra electrostatic capacity produces problems such as the apparent power is increased and the driving circuit is required to have a large current capacity, and that, when a voltage is applied to the lead pattern, piezoelectric displacement or heat generation is caused although the region is a wiring region, whereby the lead pattern formed on the surface is broken or the film is stripped.